The present invention is related to current control in general and in particular to bipolar constant current control in the drive of step motors.
Switch mode current control is a common way of controlling current. Prior art current control systems convert the current to be controlled to a voltage drop across a resistor, a comparator compares this voltage with a reference voltage, and the output of the comparator is brought to control a switch that is pulse width modulated in such a way that the average value of a series of current pulses gives the desired value.
Switching regulators are becoming more and more common. Their particular advantages over series regulators are their high efficiency. In "Computer Design" March 1978 page 103 and following, the disclosure of which is incorporated herein, a universal switching regulator is described. Such a regulator has a reference voltage, a comparator, an oscillator, an operational amplifier, an and-gate, a flip-flop, a switching transistor and a diode.
Simpler prior art systems with voltage regulation such as Lee U.S. Pat. No. 3,435,320 are compleatly inadequate for application in step motor drives since they require a plurality of windings to accomplish the switching of the transistors involved. And step motors are just not equipped with such windings. Furthermore one needs current control not voltage control.
A disadvantage of adequate prior art switching regulators is their relative complexity, which is particularly true in the case when applied to switch mode constant current control in step motor drives. In such applications, current is switched also in a direction through the coils. This is done in order to drive the motor in a given direction and at a given speed. One is here consequently concerned with two different and unrelated types of switching. One is the switchmode current control of the average current through the coils and the other is the direction of the current through the coils for the stepping of the motor.
Bipolar constant current control of step motors is described in a publication by Philips, Amsterdam, Holland, entitled "Electric Motors and Accessoires", April 1977, the disclosure of which is incorporated herein. A drive according to this publication has 46 diodes, 35 transistors, 12 capacitors and 84 resistors.